Changing views on cannabis use subsequent to decriminalization, medicalization and in some cases legalization is likely to increase perceptions of safety for use by pregnant women. The endocannabinoids are the endogenous ligands for the CB1 and CB2 receptors on which components of cannabis act to alter neural function. The role of endocannabinoids in mediating brain development is poorly understood, yet these neural modulators are ubiquitously distributed and expressed relatively early in fetal life. Sex differences in the brain are also established early in life and endocannabinoids have been implicated as essential mediators of cell number in the developing amygdala in the laboratory rat animal model. More recently, microglia, specialized immune cells of the brain, have also been implicated as essential mediators of sex differences in the brain. These observations led us to explore whether endocannabinoids modulate microglial activity and resulted in the surprising discovery of sex differences in phagocytic activity in the developing amygdala that is mediated by CB1 (and possibly CB2) receptor activation. Even more surprising, identification of the contents of microglial phagocytic cups suggests the engulfment of both mature and newly born live neurons. Thus microglia appear to be controlling both cell number and phenotype in a sex-specific manner, with the source of the sex difference being endocannabinoid tone. We have also observed a sex difference in cell genesis in the hippocampus but in the opposite direction to that of the amygdala. Endocannabiniods are integral to hippocampal functioning, much of which is different in males and females, but whether this relates to cell genesis is unknown. We now propose to build on these novel observations via completion of three specific aims which will 1) fully characterize the effect of endocannabinoids on microglial phagocytic activity in the developing amygdala and hippocampus of male and female rat pups, 2) more thoroughly identify the cellular substrates being engulfed by microglia, and 3) establish a correlation between endocannabinoid mediated phagocytosis and behaviors which are controlled by the amygdala or hippocampus and impacted by sex. These include juvenile social play behavior, stress and anxiety responding and aspects of learning and memory. Approaches used include immunohistochemistry, cell birth dating, Flow cytometery, drug administration, and extensive behavioral testing. Completion of these studies will elucidate the role of endocannabinoids in development of two key brain areas, the hippocampus and amygdala, and provide the foundations for future work on cannabis use during pregnancy and its impact on the developing brain of both males and females.